We can treat adsorption from a mixture of gases (at low pressures) by extending the Langmuir isotherm:
These reactions are not independent because both species compete for the same
surface sites. The two equilibria must be solved simultaneously:
As for the one-component case, we define surface coverages
and , and note that
and dividing, we get
The ratio of A:B on the surface is therefore different than the ratio of A:B in the gas phase,
by precisely , so the more strongly adsorbed component is enriched on the surface.
This is referred to as the thermodynamic selectivity, the
preference of the surface for one species of a mixture, and is the basis for many techniques to
separate gas mixtures.